1. Field of the Invention
The present invention relates to an apparatus for and a method of digitalizing color image data of a color image according to a dither method so that the color image can be displayed in intermediate tone by a binary display device.
2. Description of Related Art
In general, in a digitally controlled CRT display device or printer, or a digital copying machine, an image is composed of a plurality of dots of uniform size whose coordinates are digitally given, and each of those dots is indicated by a binary code of white or black (color). As a method of displaying an intermediate tone of the image with any one of those devices, a dither method is well known in the art.
According to the dither method, image data of the image of intermediate tone is digitalized in reference to threshold values which are represented by suspected random numbers. Even though the image data is added with the suspected random numbers and then digitalized, the image data remains equivalent. The dither method generally in use is an organized dither method in which a matrix of dither patterns is cyclically used as the suspected random numbers.
Where the image is in color, the color image is divided into color image data each representative of one of the colors used in the color image and the above described dither method is then applied to each color image data. In such case, in order to avoid any possible generation of Moire pattern which would result from an error in position of the dots of each color or by any other reason, different dither patterns are used for those color image data.
According to the prior art image processing apparatus in which the image data are digitalized according to the dither method, such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 60-180370 published Sep. 14, 1985, it is necessary to accomplish a digitalization (which may be referred to as a "dither processing") at a high speed on a real-time basis in reference to the dither patterns and, therefore, the dither patterns are stored in a bipolar read-only memory so that during the dither processing the dither pattern can be read out from the bipolar read-only memory.
It has, however, been found that, although the bipolar read-only memory operated at high speed, the electric power consumption thereof is considerable and, therefore, the storage of the plural dither patterns in the bipolar read-only memory only for the purpose of the dither processing of the color image tends to pose problems associated with the electric power source and the price.